The common fruit fly Drosophila melanogaster has been the
subject of a vast amount of research during the past century, and is probably
the best studied eukaryotic organism from a genetic point of view. Large
numbers of mutants have been described from nature or produced in the laboratory,
and the phenotypic abnormality of these mutants ranges from the biochemical
to the behavioral. The family Drosophilidae (what does this name mean?)
is a large one with much structural, behavioral, and ecological diversity.
The most common ecological pattern in fruit flies is for larvae and adults to feed on bacteria or yeasts that inhabit decaying vegetable matter. The range of plant products used is considerable, and includes typical fleshy fruits (apples, bananas, pears), other types of "fruit" (strawberries, tomatoes, melons), and decaying fleshy plant tissues of various types (such as succulents like cacti). The deserts of the southwestern US and Mexico are home to a large number of "cactophilic" species whose larvae live in cactus "rots" (places where the flesh of a cactus is decaying) or in dead and rotting cacti. The Hawaiian Islands are home to about 300 described species, and taxonomists believe there may be as many as 500 more awaiting description. These species are ecologically diverse; many utilize as a food source the bark of various native shrubs and trees, because there are no native fleshy fruits in Hawaii. Some species live on fungi rather than plants, and a few are predaceous as larvae. In general, the adults mate at the food substrate, and eggs are laid in the plant tissue. The larvae bore through the flesh, and feed on yeasts and plant juices. After several instars, the larvae pupate and eventually eclose as adults. Adults usually disperse before mating and ovipositing.
Drosophila females are typical insects in that they possess an elaborate reproductive system that includes sperm acceptance and storage structures (the copulatory bursa and spermatheca), in addition to the more "standard" (in vertebrate terms) structures such as a vagina, oviducts, and ovaries. Females usually mate repeatedly during their reproductive life. There is evidence that fecundity is increased by repetitious mating, and that overall fertility of the eggs is enhanced also. Males may contribute, at the time of insemination, nutritious substances along with the sperm. This material may be metabolized by the female and used to produce eggs. Fertility may be increased by repeated mating due to the presence of a greater proportion of viable sperm in "new" ejaculates.
Drosophila males have an elaborate courtship ritual that includes species specific postures, displays, and songs. Males vigorously court any available female, and mate whenever the opportunity arises. I have not heard any reports of direct physical combat between D. melanogaster males, although males of other species engage in various forms of direct reproductive competition. Because females store sperm, one would expect some form of sperm competition among males. There are a variety of possible forms of this competition, but the only one that I have heard about for D. melanogaster is sperm displacement and "last male" advantage (last sperm in is the first sperm out).
It is common for females of a wide variety of species to exercise some choice among males as potential mates. They certainly discriminate between heterospecific and conspecific males, and usually between males that differ in their "resource holding potential" (for instance between males with good vs. poor territories), or in their apparent ability or willingness to provide resources for the female and her offspring, as evidenced by various forms of nuptial gifts (as in eastern bluebirds, blackheaded terns, and hangingflies). Females may also discriminate between males that are subtly different in morphological, physiological, or behavioral characteristics (such as in peacocks, barn swallows, and jungle fowl).
Our purpose in this lab will be to perform some simple observations to get an idea about the dynamics of Drosophila courtship and mating behavior. We will assess whether females of D. melanogaster exhibit differences in receptivity to phenotypically different males, which will tell us indirectly whether males of different phenotypes differ in their ability to produce courtship displays. We might expect wild-type males to be preferred over mutant males (such as the period mutant). We might also expect some mutant types to be preferred over others. For instance, wingless males (a serious morphological deformity) might be at a grave disadvantage relative to males with eye color mutations. Another body of evidence indicates that Drosophila females prefer to mate with males that are relatively rare in the population, so that some mutant males (such as those with relatively benign mutations) might actually be at an advantage over wild-type males in simultaneous choice situations.
Handling and observing small, active, and volant animals can be tricky. Fortunately, many techniques have been worked out for fruit flies, as they have been used in many experimental studies. Flies are cultured in small milk bottles with a concoction of food in the bottom. We will use small shell vials rather than the larger milk bottles, and will not try to culture any flies, as we are mainly interested in their mating behavior. Flies can be etherized to observe them close-up, or to separate females from males (how do we sex fruit flies?).
Vials containing individuals (separated by gender) of several different mutant types will be available in the laboratory. The vials are stoppered with cotton balls, so that we can remove some individuals with a small aspirator. We will introduce flies into our mating arenas, which are plexiglass containers with a 130 x 7 mm circular area in which the behavior of the flies can be observed. It is possible to observe the behavior of the flies without a microscope, but we will have some scopes available for you to look at the morphology of the mutant flies.
There are a large number of experiments that we could do, so we will
want to narrow the field of possibilities some. We might use only wild-type
(normal) females, and compare latency to court and to mate of a variety
of mutant males. Females could be offered different males in sequence,
or be given a simultaneous choice. Given the time constraints at this time
of the semester, we will not try to collect any quantitative data during
this lab exercise, but will concentrate on observing the behavior of the
flies. What is the nature of the male courtship display? How long must
a male court before a female will mate with him? What female behaviors
signal either rejection or willingness to mate? How long does copulation
last? Are there any aggressive interactions between males?